Toward High-Performance Poly(l-lactide) Fibers via Tailoring

May 31, 2016 - As a sustainable alternative to conventional petrochemical-based polymers, biobased and biodegradable poly(l-lactide) (PLLA) exhibits ...
0 downloads 0 Views 10MB Size
Download PDF
Research Article pubs.acs.org/journal/ascecg

Toward High-Performance Poly(L‑lactide) Fibers via Tailoring Crystallization with the Aid of Fibrillar Nucleating Agent Huixian zhang, Hongwei Bai,* Zhenwei Liu, Qin Zhang, and Qiang Fu* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu 610065, People’s Republic of China S Supporting Information *

ABSTRACT: As a sustainable alternative to conventional petrochemical-based polymers, biobased and biodegradable poly(L-lactide) (PLLA) exhibits tremendous application potential in the textile industry due to its attractive elastic recovery, moisture regain, and flammability. However, the commercial adoption of PLLA textile fibers still faces some hurdles mainly associated with their poor heat resistance (i.e., high thermal shrinkage or low dimensional stability) because the low crystallization rate makes PLLA difficult to crystallize during melt spinning. Herein, we report a simple but robust strategy to address this hurdle via simultaneously manipulating crystallinity and lamellae orientation with the aid of a highly active nucleating agent (NA) that can be completely dissolved in PLLA melt and reorganize into fine fibrils upon cooling. By taking full advantage of strong elongational flow field involved in the melt spinning, the NA fibrils with high nucleation efficiency on PLLA crystallization tend to align along the flow direction and subsequently serve as nucleation templates to induce the growth of kebab-like PLLA lamellae perpendicular to their long axis, finally forming large amounts of highly orientated crystal structure in melt-spun PLLA fibers. In this way, the crystallization manipulation imparts the PLLA fibers with an impressive combination of superior mechanical strength and heat resistance. Compared with neat PLLA fiber, a prominent increase of 78% in tensile strength and a substantial decline of 1069% in boiling water shrinkage are achieved in the fiber nucleated with 0.3 wt % NA. This work could open up an avenue toward the design and development of high-performance PLLA fibers by using fibrillar nucleating agent as a nucleation template to tailor effectively crystallization in the melt spinning process. KEYWORDS: Poly(L-lactide), Fiber, Nucleating agent, Crystallization, Heat resistance

■

(PET) fibers in modern industry and life.7−9 In comparison with these petrochemical-based fibers, PLLA fibers possess not only exceptional sustainability but also some advantageous physical properties, such as better elastic recovery, higher moisture regain, better weathering stability, as well as lower flammability and smoke generation.7,8,10 Various types of commercial PLLA fibers including monofilaments, multifilaments, staple fibers, short-cut fibers, and spunbond fabrics have been fabricated using conventional melt spinning technology.4,8 Unfortunately, the application potential of PLLA fibers has not been fully realized until now, mostly because the very slow crystallization rate of PLLA makes it hard to obtain fibers with a high crystallinity (e.g., 55−60%) even under intensive elongational flow conditions (which can notably accelerate the crystallization kinetics of semicrystalline polymers by orders of magnitude relative to the quiescent condition11−13) involved in melt spinning.8,14,15 In this case, the low crystallinity (i.e.,

INTRODUCTION Over the past several decades, there has been a sustained interest in developing biodegradable polymers derived from renewable bioresources as a solution to the growing environmental sustainability issues of conventional petrochemicalbased polymers.1−3 Among these emerging biobased and biodegradable polymers, poly(L-lactide) (PLLA) is the most promising frontrunner with 100% renewability from abundant plant resources (e.g., corn and sugar), favorable biocompatibility, appealing mechanical properties, excellent transparency, and good processability.4−6 The starting application of PLLA is mainly focused on the high-value biomedical sector owing to its high initial production cost. Since the beginning of truly industrial production in the 1990s, its commercial introduction as a commodity thermoplastic has been gradually expanded to more general applications with the remarkable drop of price, particularly in the short-time packaging industry.2,6 Recently, PLLA textile fibers with tremendous application value and market potential in various fields have garnered considerable enthusiasm as an eco-friendly alternative to the most widely used nylon and poly(ethylene terephthalate) © XXXX American Chemical Society

Received: April 16, 2016 Revised: May 24, 2016

A

DOI: 10.1021/acssuschemeng.6b00784 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering 20−30%) cannot provide the melt-spun PLLA fibers with sufficient heat resistance for ironing and dyeing (both of them typically take place above 120 °C) due to the limitation of relatively low glass transition temperature (Tg) of around 55− 60 °C.8,15,16 On the other hand, the insufficient heat resistance also significantly hampers their use in numerous environments where low thermal shrinkage and high dimensional stability are required. Thermal annealing provides a straightforward pathway to improve the heat resistance of PLLA fibers via enhancing crystallinity, but it is time-consuming and only limited improvement has been obtained.17 Actually, for semicrystalline polymers consisting of alternating crystalline and amorphous phases, their performance is also strongly dependent on crystal structure, crystal morphology, and lamellae orientation.7,18−25 Formation of an oriented crystal structure is favorable for improving the performance of polymers.7,18,19,22,25 Imposing an intensive flow field seems to be a facile method to trigger dramatically enhanced crystallization kinetics as well as highly oriented crystallization of polyolefin.13,22,25 Nevertheless, for semirigid PLLA chains, flow conditions involved in conventional melt spinning at low speeds (usually 200−1000 m/min) cannot meet the essential requirement for highly oriented crystallization.8,19,26 Thus, achieving melt-spun PLLA fibers with excellent heat resistance via tailoring crystallization becomes an urgent challenge to be addressed. To date, a great deal of effort has been devoted to meet such a challenge.7,8,14,26 In particular, high take-up speed (e.g., 2000−5000 m/min)7,8,26 and hot drawing7,8,14 are frequently used to improve the heat resistance and mechanical properties of melt-spun PLLA fibers. Both the crystallinity and lamellae orientation are found to increase significantly with increasing take-up speed and draw ratio due to the flow-induced crystallization.7,8,14,26 By adjusting the take-up speed in highspeed melt spinning and subsequent draw ratio in hot drawing, substantially improved mechanical strength and heat resistance have been successfully achieved in the PLLA fibers.26 However, although high-speed melt spinning offers a great opportunity toward heat-resistant PLLA fibers, fabricating the fibers with uniform diameter is still not a trivial task under the high-speed condition due to the draw instability caused by spinline necklike deformation, which is characterized by an abrupt drop of spinline cross-sectional area.4,27 Besides giving rise to higher crystallinity and crystal orientation, this instable deformation could lead to a frequent fiber breakage in hot drawing and even deteriorate the fiber properties. Therefore, a more universal and powerful strategy is still highly desirable for realizing industrial fabrication of high-performance PLLA fibers at low spinning speeds. Addition of a nucleating agent (NA) including polylactide stereocomplex and organic small molecules offers the most efficient and handy way to enhance significantly crystallization rate and even control effectively crystal morphology of semicrystalline polymers by providing large amounts of heterogeneous nucleation centers with tunable shapes.28−44 For example, some small molecular organic NAs for PLLA crystallization, such as N,N′,N″-tricyclohexyl-1,3,5-benzenetricarboxylamide (TMC-328)28−30 and tetramethylene-dicarboxylic dibenzoyl-hydrazide (TMC-306),45 can be dissolved in PLLA melt and self-organize into fibrils upon cooling to act as effective nucleation templates for the epitaxial growth of kebablike PLLA lamellae on their surface along the direction perpendicular to the long axis, forming a shish-kebab-like

crystal structure. Very recently, we demonstrated that these fibrils self-organized in PLLA melt have a strong tendency to align along the shear flow direction and such highly ordered alignment of NA fibrils can readily transform into the high orientation of PLLA lamellae during subsequent surfacenucleated crystallization.29,45 Moreover, the nucleation efficiency of NAs could be further enhanced under flow conditions experienced in melt processing operations.36,46,47 These inspiring results suggest that the presence of NAs capable of self-organizing into fibrils in melt could effectively tailor crystallization of PLLA during melt spinning. However, to the best of our knowledge, no attention has been paid to this issue until now. It is unclear whether the rapid cooling of PLLA melt may provide sufficient time for the self-organization of NAs and the subsequent development of NA-tailored PLLA crystallization. In this work, the strategy of introducing trace amounts (0.2−0.5 wt %) of fibrillar TMC-306 as a robust nucleation template is therefore naturally proposed to simultaneously manipulate crystallization kinetics and lamellae orientation of PLLA, aiming to fabricate high-performance PLLA fibers with superior heat resistance through conventional low-speed melt spinning technology. Compared with TMC-328, the TMC-306 molecules have a better solubility in PLLA melt.28,45 It was expected that the intensive elongational flow field of low-speed melt spinning could facilitate the alignment of TMC-306 fibrils along the spinning direction and, subsequently, the highly aligned TMC-306 fibrils could induce the formation of highly oriented PLLA lamellae during the solidification of the PLLA fibers with a significantly enhanced crystallinity (Scheme 1). Scheme 1. Schematic Illustration Showing the Morphological Evolution of PLLA/TMC-306 Mixtures during Isothermal Crystallization under Flowa

a

(a) Well-dispersed TMC-306 within PLLA matrix, (b) TMC-306 molecules self-organize into fibrils, (c) flow-induced alignment of TMC-306 fibrils, and (d) epitaxial crystallization of PLLA on the fibril surface yields a highly oriented PLLA lamellar structure. The flow direction is horizontal.

The results confirm that the introduction of TMC-306 makes it possible to fabricate high-performance PLLA fibers with a high crystallinity (ca. 55−60%) and a highly oriented PLLA lamellae using conventional low-speed melt spinning machines. Both the substantially improved heat resistance and mechanical properties of the nucleated PLLA fibers have been demonstrated for the first time by comparison with neat PLLA fiber. The B

DOI: 10.1021/acssuschemeng.6b00784 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) Schematic representation of melt spinning setup: (1) hopper, (2) extruder, (3) screw, (4) metering pump, (5) mesh filters and distributor, (6) spinneret, (7) applicator roller, (8) take-up godet, (9) fast godet, (10) idler roller, (11) winder, and (12) heater. (b) Morphology of melt-spun PLLA fibers.

influence of TMC-306 loading on fiber performance is also discussed. We postulate that the strategy developed in our work could enable the large-scale fabrication of high-performance PLLA fibers using melt spinning technology to meet the demands of the PLLA textile market.

■

micrographs were recorded with a Cannon camera during the cooling and subsequent isothermal crystallization processes. Differential Scanning Calorimetry (DSC). Thermal properties were measured using a PerkineElmer pyris-1 DSC (USA) under dry nitrogen atmosphere. About 5 mg specimens cut from PLLA fibers were heated from 30 to 200 °C at a rate of 10 °C/min and the crystallinity (Xc) of the fibers was evaluated by subtracting the cold crystallization enthalpy from the melting enthalpy according to the following equation:

EXPERIMENTAL SECTION

Materials and Sample Preparation. Poly(L-lactide) (PLLA, grade 4032D) with a weight-averaged molecular weight (Mw) of 1.7 × 105 g/mol and polydispersity of 1.74 was obtained from Natureworks, USA. Its melting temperature (Tm) was determined to be 168 °C by differential scanning calorimetery (DSC). Tetramethylenedicarboxylic dibenzoylhydrazide (trade name TMC-306) selected as the highly active nucleating agent for PLLA crystallization was kindly provided by Shanxi Provincial Institute of Chemical Industry, China. According to the product information provided by the producer, the TMC-306 is nontoxic and has no risk to human health. PLLA without and with trace amounts (0.1−0.5 wt %) of TMC-306 were prepared by extrusion using a corotating twin-screw extruder (TSSJ-25) at a screw speed of 120 rpm. The barrel temperatures were set as 160−190 °C from hopper to die. Note that, to achieve precise loading and good dispersion of such small amounts of TMC-306 in PLLA, a master-batch of PLLA/TMC-306 (95/5, wt/wt) was first prepared and then diluted with neat PLLA. The obtained PLLA/ TMC-306 mixtures were abbreviated as PLLA-x, where “x” indicates the weight percentage of TMC-306 in the mixture. After pelletizing, the fibers of these mixtures were spun using a melt spinning setup with a multifilament spinneret (18 holes with a diameter of 0.4 mm), as schematically shown in Figure 1a. This is a two-stage melt spinning process: the mixtures were first melt extruded through the spinneret at 190 °C, followed by collecting with a take-up speed of 150 m/min at room temperature, and then the as-spun fibers undergo hot-drawing (between the take-up godet and the fast godet) with a draw ratio of around 2.5 at 100 °C. The diameter of the melt-spun fibers is about 29 μm (Figure 1b). To avoid any excessive degradation caused by hydrolysis, all materials were dried in a vacuum at 60 °C for 24 h before extrusion and melt spinning. Polarized Optical Microscopy (POM). A Leica DMLP polarization optical microscope (POM) equipped with a Linkam CSS-450 shearing hot stage was used to observe the morphological evolution of PLLA crystals during isothermal crystallization under quiescent and shear conditions. Specimens with a thickness of about 50 μm were sandwiched between two quartz windows in the Linkam CSS-450 hot stage. After any thermal history was erased at 200 °C, specimens were cooled to the temperature of 145 °C at a rate of 30 °C/min and then subjected to a step shear flow (20 s−1 for 1 s). POM observations were performed just after the cooling from the melts, and the POM

Xc =

ΔHm wf ΔHmo

(1)

where ΔHm is the measured melt enthalpy during the DSC heating scan, and ΔHom is the melting enthalpy of 100% crystalline PLLA (selected as 93.6 J/g48). Wide-Angle X-ray Diffraction (WAXD). One-dimensional wideangle X-ray diffraction (1D-WAXD) patterns were recorded using a X’ Pert Pro MPD (Philips, Netherlands) diffractmeter equipped with a Ni-filtered Cu Kα radiation (40 kV and 40 mA). Scans were made between Bragg angles of 0°−40°. Two-dimensional wide-angle X-ray diffraction (2D-WAXD) experiments were carried out using a Bruker D8 Discover X-ray diffractometer equipped with a Vantec 500 detector. The sample to detector distance was 8.3 cm. Specimens of fiber bundles were placed perpendicular to the beam. The crystal orientation of PLLA fibers was quantified on the basis of Herman’s orientation parameter ( f) using the following equations: f=

3⟨cos2 φ⟩ − 1 2 2

⟨cos φ⟩ =

∫0

π /2

∫0

(2)

I(ϕ)sin ϕcos2 ϕdϕ π /2

I(ϕ)sin ϕdϕ

(3)

where φ is the angle between the normal of a given crystal plane and the fiber direction; I is the corresponding integral intensity. Tensile Testing. Tensile mechanical properties were measured at room temperature using an YG001A electronic monofilament strength tester (Textile Instruments Factory, China) at a cross-bead speed of 20 mm/min. For each fiber sample, the reported properties were obtained from at least 25 independent specimens with a gauge length of 20 mm. Thermal Shrinkage. Thermal shrinkage was tested in boiling water and hot air, respectively. During the testing of boiling water shrinkage (BWS), fiber specimens packed in the cotton gauze were immersed in boiling water for 30 min without any tension and then kept at room temperature for 20 min. The shrinkage in hot air was measured by heating the fiber specimens in an oven with a temperature of 140 °C for 30 min without any tension. For each sample, at least six independent specimens were tested. C

DOI: 10.1021/acssuschemeng.6b00784 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. POM micrographs showing the crystal morphology evolution of PLLA and PLLA-0.3 during isothermal crystallization at 145 °C after being sheared at 20 s−1 for 1 s. The crystallization time is given in each micrograph and the yellow arrow indicates the shear flow direction.

intensive shear or/and elongational flow fields experienced in melt processing, crystal morphology evolution of PLLA and PLLA/TMC-306 mixtures during isothermal crystallization under shear conditions was comparatively investigated using POM. In each case, the samples were cooled rapidly from the melt state (200 °C) to a selected temperature of 145 °C and then a step shear flow (20 s−1 for 1 s) was applied, after which the samples were held at this temperature for a given time until PLLA crystallization is complete. Some representative POM micrographs of PLLA and PLLA-0.3 are shown in Figure 2. As expected, although PLLA chains might be orientated under the applied shear condition, their relaxation time is too short to induce any highly orientated crystal structure and thus highly oriented PLLA lamellar structure cannot be seen in neat PLLA (Figure 2a).19,49 Besides isotropic PLLA spherulites usually formed under quiescent crystallization condition (Figure 3a),

The thermal shrinkage was calculated according to the following relationship: shrinkage (%) =

(L0 − L) × 100% L0

(4)

where L0 is the initial length of the specimens before testing, and L is the final length of the fully shrunk specimens after testing, respectively.

■

RESULTS AND DISCUSSION Flow-Induced Alignment of Nucleating Agent Fibrils and Subsequent Directing Crystallization of PLLA Lamellae on the Fibril Surface. As mentioned above, TMC-306 molecules can be dissolved in PLLA melt and reorganize into fibrils upon cooling to act as effective nucleation templates for PLLA crystallization. To illuminate the role of the TMC-306 fibrils in directing crystallization of PLLA under D

DOI: 10.1021/acssuschemeng.6b00784 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 3. POM micrographs showing the crystal morphology of PLLA and PLLA-0.3 during isothermal crystallization at 145 °C under quiescent condition. The crystallization time is given in each micrograph.

Figure 4. (a) DSC melting curves and (b) crystallinity for melt-spun PLLA fibers with various amounts of TMC-306.

Crystallinity and Lamellae Orientation of Melt-Spun PLLA Fibers. PLLA fibers without and with TMC-306 were fabricated using low-speed (150 m/min) melt spinning method. Because elongational fields are considered to be much more effective in triggering orientated structure in polymer melts as compared with the shear flow, it is natural to expect that the elongational flow involved in the melt spinning would facilitate the well-ordered alignment of TMC-306 fibrils in PLLA melt along the flow direction and these aligned fibrils would act as efficient nucleation templates to tailor subsequent PLLA crystallization under rapid cooling conditions. Figure 4a shows the DSC melting curves of the melt-spun PLLA fibers with and without TMC-306. There are two endothermic peaks in the thermograms of some nucleated PLLA fibers. The double melting behavior of PLLA is usually attributed to the melting− recrystallization-remelting process upon heating.34,50 The peak at lower temperature is believed to be the melting of original PLLA crystals, whereas the other one at higher temperature is associated with the melting of recrystallized PLLA crystals with thicker and more perfect lamellae. The scanning rate dependence is regarded as an evidence for the melting− recrystallization mechanism.51,52 Integrated intensity of the low melting peak relative to the high melting one is generally increased evidently with increasing scanning rate. However, an

only small amounts of row-like crystal structure can be observed (Figure 2a3). However, the crystallization habit of PLLA changes dramatically with the introduction of TMC-306. For PLLA-0.3, the TMC-306 molecules completely dissolved in PLLA melt (Figure 2b1) can easily self-organize into fibrils upon cooling (Figure 2b2) and such fibrils exhibit a strong tendency to be aligned with the step shear flow due to the large aspect ratio (Figure 2b3). Because the relaxation time of TMC306 fibrils is very long, the subsequent epitaxial growth of kebab-like PLLA lamellae on the surface of these well-aligned fibrils yields a highly oriented PLLA lamellar structure (Figure 2b4). In contrast, only randomly arranged shish-kebab-like crystals can be observed under quiescent condition (Figure 3). Furthermore, PLLA-0.3 displays a much faster crystallization rate relative to the neat PLLA under both the two crystallization conditions (Figure 2a4,b4; Figure 3a2,b2). Moreover, the TMC-306 fibrils and shear flow exhibit a notable synergistic effect on the enhancement of PLLA crystallization rate (Figures 2 and 3). These results clearly confirm that TMC-306 can not only significantly accelerate PLLA crystallization by proving heterogeneous nucleating sites but also effectively manipulate crystal orientation by serving as well-aligned nucleation template under flow conditions. E

DOI: 10.1021/acssuschemeng.6b00784 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

temperatures could arise from the melting of α′- and α-form crystals, respectively. The reason for the formation of α′-form crystals in PLLA fibers with the introduction of TMC-306 is still needed to be explored in our future work. The value of PLLA crystallinity (Xc) of PLLA fibers increases substantially from 35% to 57% as TMC-306 loading increases from 0 to 0.2 wt % (Figure 4b), demonstrating that TMC-306 can act as a high active NA to accelerate significantly PLLA crystallization kinetics during the melt spinning. However, when TMC-306 loading exceeds 0.2 wt %, no further steady increase in Xc can be obtained in the nucleated fibers because its nucleating efficiency reaches saturation.28,45 2D-WAXD analysis provides a clear-cut evidence for the formation of highly oriented crystal structure in the melt-spun PLLA fibers. As presented in Figure 6, the two-arc 2D-WAXD patterns in the meridional direction reveal the preferred orientation of PLLA lamellae perpendicular to the elongational flow direction or the fiber axis as expected. For the purpose of comparison, the Herman’s orientation parameter ( f) is calculated for each sample and the results are plotted in Figure 7. Obviously, the TMC-306 fibrils can template a highly

opposite change tendency is observed in the endotherms of the nucleated PLLA fibers (Figure S1), suggesting that other mechanisms should be considered to explain their double melting behaviors. Because PLLA is a typical polymorphic polymer, the existence of dual crystal structures may also cause the appearance of double melting peaks.53−55 Thus, the crystal structure of the melt-spun PLLA fibers was characterized with 1D-WAXD and the obtained 1D-WAXD patterns are shown in Figure 5. Noticeably, both the neat PLLA and PLLA-0 have the

Figure 5. 1D-WAXD patterns for melt-spun PLLA fibers.

same characteristic diffraction peaks at 2θ values of 16.7° and 19.0°, corresponding to the (200)/(110) and (203) planes of α-form crystals, respectively. But for the PLLA fibers containing 0.2−0.5 wt % TMC-306, a small but notable peak shifting in the 2θ positions is found for both the two strong diffraction peaks, implying the existence of some disordered α′-form crystals in these fibers.55 It has been proved that both the αand α′-form crystals share the same 103 helix chain conformation and orthorhombic unit cell, but the packing of the side groups in the helical chains of the α′-form crystals is less ordered and looser than that of the α-form crystals.53 The endotherms of the nucleated PLLA fibers at lower and higher

Figure 7. Herman’s orientation parameter ( f) of melt-spun PLLA fibers with various amounts of TMC-306 obtained from 2D-WAXD.

Figure 6. 2D-WAXD patterns for melt-spun PLLA fibers: (a) PLLA-0, (b) PLLA-0.1, (c) PLLA-0.2, (d) PLLA-0.3, and (e) PLLA-0.5. The fiber axis is horizontal. F

DOI: 10.1021/acssuschemeng.6b00784 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

nucleated PLLA fibers exhibit maxima at a TMC-306 loading of 0.3 wt %. Boiling water shrinkage (BWS) and shrinkage in hot air (140 °C) were measured to evaluate the heat resistance of the meltspun PLLA fibers, and the results are shown in Figure 9. It is interesting to observe that both the BWS and shrinkage at 140 °C decrease significantly with the introduction of 0.1−0.5 wt % TMC-306, indicating that the TMC-306 fibrils tailored PLLA crystallization can impart the fibers with a superior heat resistance. More interestingly, the heat resistance is also dominated by the PLLA lamellae orientation, as that for tensile properties. The heat resistance of the fibers enhances substantially as the Herman’s orientation parameter of PLLA lamellae increases, and thus PLLA-0.3 fiber exhibits the highest heat resistance. In particular, the BWS of PLLA-0.3 fiber is as low as 0.67%, in comparison with 7.83% of the neat PLLA fiber. On the basis of the above discussion, it is very clear that the melt-spun PLLA fibers with high mechanical strength and heat resistance can be fabricated by using fibrillar NAs to manipulate simultaneously crystallinity and lamellae orientation under elongational flow fields. The lamellae orientation is postulated to play a key role in enhancing the fiber performance. The substantial enhancement in mechanical properties and thermal shrinkage of PLLA fibers with increasing lamellae orientation could originate from the interlocking effect between parallelaligned PLLA lamellae. Distinct from the randomly arranged lamellae, the parallel-aligned lamellae could readily interlock with each other at interfaces,29,56 thus forming a densely packed interlocked structure to resist mechanical deformation and thermal shrinkage of PLLA fibers during practical applications. Considering that melt spinning is a fast process, in which PLLA melt is continuously drawn from a spinneret, the spinning conditions (such as melting temperature, take-up speed, and draw ratio) have significant influence on the selforganization of NA molecules into fibrils, alignment of fibrils in PLLA melt, and subsequent crystallization of PLLA on fibril surface. More attention will be focused on the optimization of melt spinning conditions in our future work, to optimize further the performance of PLLA fibers.

oriented PLLA lamellar structure during the melt spinning process and the degree of lamellae orientation is strongly dependent on the TMC-306 loading. Increasing TMC-306 loading from 0 to 0.3 wt % leads to a substantial enhancement in f, but an evidently declined f is observed with further increasing its loading up to 0.5 wt %. Specially, the f of PLLA0.5 fiber is comparable to that of the PLA-0.2. This phenomenon can be explained by the difference in the length of TMC-306 fibrils. Comparing with long TMC-306 fibrils selforganized in PLLA-0.3 melt, the short fibrils in PLLA-0.5 caused by the decreased solubility of TMC-306 at higher loading are more difficult to be aligned with the flow.45 Similar results have also been reported in PLLA/TMC-328 mixtures.28 Mechanical Properties and Heat Resistance of MeltSpun PLLA Fibers. In the pursuit of high-performance polymer fibers, previous experimental and theoretical investigations have proved that both the crystallinity and lamellae orientation play a critical role in enhancing the fiber performance.4,7,8 To explore the potential of the TMC-306 tailored PLLA crystallization on the performance enhancement, influence of TMC-306 loading on mechanical properties of the melt-spun PLLA fibers were investigated by uniaxial tensile testing. As presented in Figure 8, introduction of 0.1−0.5 wt %

Figure 8. Tension properties of melt-spun PLLA fibers with various amounts of TMC-306.

■

CONCLUSIONS In summary, we have demonstrated that introducing trace amounts (0.1−0.5 wt %) of highly active NA capable of selforganizing into fibrils in PLLA melt is a facial but robust strategy to fabricate mechanically strong PLLA fibers with super heat resistance through conventional low-speed melt spinning

TMC-306 into PLLA fiber gives rise to a greatly enhanced tensile strength (78%) and Young’s modulus (47%). Moreover, the obtained reinforcement efficiency is found to be dominated by the PLLA lamellae orientation. Both the reinforcement efficiency and lamellae orientation (Figure 7) follow the same changing trend with the increase of TMC-306 loading. The

Figure 9. (a) Boiling water shrinkage and (b) shrinkage in hot air of melt-spun PLLA fibers with various amounts of TMC-306. G

DOI: 10.1021/acssuschemeng.6b00784 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering technology. During the melt spinning, these NA fibrils are found to align along the elongation flow direction and subsequently act as efficient nucleation templates to induce the formation of a highly orientated PLLA lamellar structure in melt-spun PLLA fibers. In comparison with neat PLLA fiber, PLLA/NA composite fibers exhibit not only a higher mechanical strength but also a much better heat resistance because the presence of NA fibrils can simultaneously enhance PLLA crystallinity and lamellae orientation. Specially, a substantially decreased (1069%) boiling water shrinkage is obtained in the fiber containing 0.3 wt % NA. The role of NA content in enhancing the lamellae orientation and resulting fiber performance has also been revealed. We believe that these inspiring results could significantly promote the applications of PLLA as textile fibers.

■

Selke, S. E. M., Tsuji, H., Eds.; John Wiley & Sons, Inc.: Hoboken, NJ, 2010. (9) Xu, S. X.; Chen, J. G.; Wang, B. J.; Yang, Y. Q. Sustainable and hydrolysis-free dyeing process for poly(lactic acid) using nonaqueous medium. ACS Sustainable Chem. Eng. 2015, 3, 1039−1046. (10) Choi, J. H.; Seo, W. Y. Coloration of poly(lactic acid) with disperse dyes. 1. Comparison to poly(ethylene terephthalate) of dyeability, shade and fastness. Fibers Polym. 2006, 7, 270−275. (11) Li, L. B.; de Jeu, W. H. Shear-induced crystallization of poly(butylene terephthalate): A real-time small-angle X-ray scattering study. Macromolecules 2004, 37, 5646−5652. (12) Song, S. J.; Wu, P. Y.; Feng, J. C.; Ye, M. X.; Yang, Y. L. Influence of pre-shearing on the crystallization of an impact-resistant polypropylene copolymer. Polymer 2009, 50, 286−295. (13) Yang, L.; Somani, R. H.; Sics, I.; Hsiao, B. S.; Kolb, R.; Fruitwala, H.; Ong, C. Shear-induced crystallization precursor studies in model polyethylene blends by in-situ rheo-SAXS and rheo-WAXD. Macromolecules 2004, 37, 4845−4859. (14) Fambri, L.; Pegoretti, A.; Fenner, R.; Incardona, S. D.; Migliaresi, C. Biodegradable fibres of poly(L-lactic acid) produced by melt spinning. Polymer 1997, 38, 79−85. (15) John, M. J.; Anandjiwala, R.; Oksman, K.; Mathew, A. P. Meltspun polylactic acid fibers: Effect of cellulose nanowhiskers on processing and properties. J. Appl. Polym. Sci. 2013, 127, 274−281. (16) Yang, Y. Q.; Huda, S. Dyeing conditions and their effects on mechanical properties of polylactide fabric. AATCC Rev. 2003, 3, 56− 61. (17) Agrawal, A. K.; et al. Spinning of poly(lactic acid) fibers. In Poly(lactic acid): Synthesis, Structures, Properties, Processing, and Applications; Auras, R., Lim, L.-T., Selke, S. E. M., Tsuji, H., Eds.; John Wiley & Sons, Inc.: Hoboken, NJ, 2010. (18) Na, B.; Tian, N. N.; Lv, R. H.; Zou, S. F.; Xu, W. F.; Fu, Q. Annealing-induced oriented crystallization and its influence on the mechanical responses in the melt-spun monofilament of poly(Llactide). Macromolecules 2010, 43, 1156−1158. (19) Xu, H.; Zhong, G. J.; Fu, Q.; Lei, J.; Jiang, W.; Hsiao, B. S.; Li, Z. M. Formation of shish-kebabs in injection-molded poly(L-lactic acid) by application of an intense flow field. ACS Appl. Mater. Interfaces 2012, 4, 6774−6784. (20) Bai, H. W.; Luo, F.; Zhou, T. N.; Deng, H.; Wang, K.; Fu, Q. New insight on the annealing induced microstructural changes and their roles in the toughening of beta-form polypropylene. Polymer 2011, 52, 2351−2360. (21) Luo, F.; Geng, C. Z.; Wang, K.; Deng, H.; Chen, F.; Fu, Q.; Na, B. New understanding in tuning toughness of beta-polypropylene: The role of beta-nucleated crystalline morphology. Macromolecules 2009, 42, 9325−9331. (22) Su, R.; Zhang, Z. Q.; Gao, X.; Ge, Y. A.; Wang, K.; Fu, Q. Polypropylene injection molded part with novel macroscopic bamboolike bionic structure. J. Phys. Chem. B 2010, 114, 9994−10001. (23) Gan, Z. H.; Kuwabara, K.; Abe, H.; Iwata, T.; Doi, Y. The role of polymorphic crystal structure and morphology in enzymatic degradation of melt-crystallized poly(butylene adipate) films. Polym. Degrad. Stab. 2005, 87, 191−199. (24) Liu, G. M.; Zhang, X. Q.; Wang, D. J. Tailoring crystallization: Towards high-performance poly(lactic acid). Adv. Mater. 2014, 26, 6905−6911. (25) Chen, Y. H.; Zhong, G. J.; Wang, Y.; Li, Z. M.; Li, L. B. Unusual tuning of mechanical properties of isotactic polypropylene using counteraction of shear flow and beta-nucleating agent on beta-form nucleation. Macromolecules 2009, 42, 4343−4348. (26) Schmack, G.; Tandler, B.; Vogel, R.; Beyreuther, R.; Jacobsen, S.; Fritz, H. G. Biodegradable fibers of poly(L-lactide) produced by high-speed melt spinning and spin drawing. J. Appl. Polym. Sci. 1999, 73, 2785−2797. (27) Shin, D. M.; Lee, J. S.; Jung, H. W.; Hyun, J. C. High-speed fiber spinning process with spinline flow-induced crystallization and necklike deformation. Rheol. Acta 2006, 45, 575−582.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00784. DSC melting curves of the melt-spun PLLA-0.3 fiber (PDF).

■

AUTHOR INFORMATION

Corresponding Authors

*H. Bai. E-mail: [email protected], [email protected]. Tel./Fax: +86 28 8546 0953. *Q. Fu. E-mail: [email protected]. Tel./Fax: +86 28 8546 1795. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (Nos. 51421061 and 21404075), the Project of State Key Laboratory of Polymer Materials Engineering (No. sklpme2015-3-01), Science Foundation for The Excellent Youth Scholars of Sichuan University (No. 2015SCU04A28), and Scientific Research Foundation for Young Teachers of Sichuan University (No. 2015SCU11007).

■

REFERENCES

(1) Chen, G. Q.; Patel, M. K. Plastics derived from biological sources: Present and future: A technical and environmental review. Chem. Rev. 2012, 112, 2082−2099. (2) Rhim, J. W.; Park, H. M.; Ha, C. S. Bio-nanocomposites for food packaging applications. Prog. Polym. Sci. 2013, 38, 1629−1652. (3) Miller, S. A. Sustainable polymers: replacing polymers derived from fossil fuels. Polym. Chem. 2014, 5, 3117−3118. (4) Lim, L. T.; Auras, R.; Rubino, M. Processing technologies for poly(lactic acid). Prog. Polym. Sci. 2008, 33, 820−852. (5) Nampoothiri, K. M.; Nair, N. R.; John, R. P. An overview of the recent developments in polylactide (PLA) research. Bioresour. Technol. 2010, 101, 8493−8501. (6) Arrnentano, I.; Bitinis, N.; Fortunati, E.; Mattioli, S.; Rescignano, N.; Verdejo, R.; Lopez-Manchado, M. A.; Kenny, J. M. Multifunctional nanostructured PLA materials for packaging and tissue engineering. Prog. Polym. Sci. 2013, 38, 1720−1747. (7) Gupta, B.; Revagade, N.; Hilborn, J. Poly(lactic acid) fiber: An overview. Prog. Polym. Sci. 2007, 32, 455−482. (8) Mochizuki, M. Textile applications. In Poly(lactic acid): Synthesis, Structures, Properties, Processing, and Applications; Auras, R., Lim, L.-T., H

DOI: 10.1021/acssuschemeng.6b00784 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering (28) Bai, H. W.; Zhang, W. Y.; Deng, H.; Zhang, Q.; Fu, Q. Control of crystal morphology in poly(L-lactide) by adding nucleating agent. Macromolecules 2011, 44, 1233−1237. (29) Bai, H. W.; Huang, C. M.; Xiu, H.; Zhang, Q.; Deng, H.; Wang, K.; Chen, F.; Fu, Q. Significantly improving oxygen barrier properties of polylactide via constructing parallel-aligned shish-kebab-like crystals with well-interlocked boundaries. Biomacromolecules 2014, 15, 1507− 1514. (30) Gui, Z. Y.; Lu, C.; Cheng, S. J. Comparison of the effects of commercial nucleation agents on the crystallization and melting behaviour of polylactide. Polym. Test. 2013, 32, 15−21. (31) Rahman, N.; Kawai, T.; Matsuba, G.; Nishida, K.; Kanaya, T.; Watanabe, H.; Okamoto, H.; Kato, M.; Usuki, A.; Matsuda, M.; Nakajima, K.; Honma, N. Effect of polylactide stereocomplex on the crystallization behavior of poly(L-lactic acid). Macromolecules 2009, 42, 4739−4745. (32) Liu, H. L.; Bai, D. Y.; Bai, H. W.; Zhang, Q.; Fu, Q. Constructing stereocomplex structures at the interface for remarkably accelerating matrix crystallization and enhancing the mechanical properties of poly(L-lactide)/multi-walled carbon nanotube nanocomposites. J. Mater. Chem. A 2015, 3, 13835−13847. (33) Yin, H. Y.; Wei, X. F.; Bao, R. Y.; Dong, Q. X.; Liu, Z. Y.; Yang, W.; Xie, B. H.; Yang, M. B. Enhancing thermomechanical properties and heat distortion resistance of poly(L-lactide) with high crystallinity under high cooling rate. ACS Sustainable Chem. Eng. 2015, 3, 654− 661. (34) Ma, P. M.; Xu, Y. S.; Wang, D. W.; Dong, W. F.; Chen, M. Q. Rapid crystallization of poly(lactic acid) by using tailor-made oxalamide derivatives as novel soluble-type nucleating agents. Ind. Eng. Chem. Res. 2014, 53, 12888−12892. (35) Liu, Z. W.; Luo, Y. L.; Bai, H. W.; Zhang, Q.; Fu, Q. Remarkably enhanced impact toughness and heat resistance of poly(L-Lactide)/ thermoplastic polyurethane blends by constructing stereocomplex crystallites in the matrix. ACS Sustainable Chem. Eng. 2016, 4, 111− 120. (36) Bai, J.; Wang, J. Y.; Wang, W. T.; Fang, H. G.; Xu, Z. H.; Chen, X. S.; Wang, Z. G. Stereocomplex crystallite-assisted shear-induced crystallization kinetics at a high temperature for asymmetric biodegradable PLLA/PDLA blends. ACS Sustainable Chem. Eng. 2016, 4, 273−283. (37) Arias, V.; Odelius, K.; Hoglund, A.; Albertsson, A. C. Homocomposites of polylactide (PLA) with induced interfacial stereocomplex crystallites. ACS Sustainable Chem. Eng. 2015, 3, 2220−2231. (38) Fan, Y. Q.; Yu, Z. Y.; Cai, Y. H.; Hu, D. D.; Yan, S. F.; Chen, X. S.; Yin, J. B. Crystallization behavior and crystallite morphology control of poly(L-lactic acid) through N,N ′-bis(benzoyl)sebacic acid dihydrazide. Polym. Int. 2013, 62, 647−657. (39) He, D. R.; Wang, Y. M.; Shao, C. G.; Zheng, G. Q.; Li, Q.; Shen, C. Y. Effect of phthalimide as an efficient nucleating agent on the crystallization kinetics of poly(lactic acid). Polym. Test. 2013, 32, 1088−1093. (40) Kawamoto, N.; Sakai, A.; Horikoshi, T.; Urushihara, T.; Tobita, E. Nucleating agent for poly(L-lactic acid) - An optimization of chemical structure of hydrazide compound for advanced nucleation ability. J. Appl. Polym. Sci. 2007, 103, 198−203. (41) Song, P.; Chen, G. Y.; Wei, Z. Y.; Chang, Y.; Zhang, W. X.; Liang, J. C. Rapid crystallization of poly(L-lactic acid) induced by a nanoscaled zinc citrate complex as nucleating agent. Polymer 2012, 53, 4300−4309. (42) Xu, Y. T.; Wu, L. B. Synthesis of organic bisurea compounds and their roles as crystallization nucleating agents of poly(L-lactic acid). Eur. Polym. J. 2013, 49, 865−872. (43) Tsuji, H. Poly(lactide) stereocomplexes: Formation, structure, properties, degradation, and applications. Macromol. Biosci. 2005, 5, 569−597. (44) Tsuji, H. Poly(lactic acid) stereocomplexes: A decade of progress. Adv. Drug Delivery Rev. 2016, DOI: 10.1016/ j.addr.2016.04.017.

(45) Bai, H. W.; Huang, C. M.; Xiu, H.; Zhang, Q.; Fu, Q. Enhancing mechanical performance of polylactide by tailoring crystal morphology and lamellae orientation with the aid of nucleating agent. Polymer 2014, 55, 6924−6934. (46) Byelov, D.; Panine, P.; Remerie, K.; Biemond, E.; Alfonso, G. C.; de Jeu, W. H. Crystallization under shear in isotactic polypropylene containing nucleators. Polymer 2008, 49, 3076−3083. (47) D’Haese, M.; Langouche, F.; Van Puyvelde, P. On the effect of particle size, shape, concentration, and aggregation on the flowinduced crystallization of polymers. Macromolecules 2013, 46, 3425− 3434. (48) Garlotta, D. A literature review of poly(lactic acid). J. Polym. Environ. 2001, 9, 63−84. (49) Tang, H.; Chen, J. B.; Wang, Y.; Xu, J. Z.; Hsiao, B. S.; Zhong, G. J.; Li, Z. M. Shear flow and carbon nanotubes synergistically induced nonisothermal crystallization of poly(lactic acid) and its application in injection molding. Biomacromolecules 2012, 13, 3858− 3867. (50) Wang, S. S.; Han, C. Y.; Bian, J. J.; Han, L. J.; Wang, X. M.; Dong, L. S. Morphology, crystallization and enzymatic hydrolysis of poly(L-lactide) nucleated using layered metal phosphonates. Polym. Int. 2011, 60, 284−295. (51) Lee, Y. C.; Porter, R. S.; Lin, J. S. On the double-melting behavior of poly(Ether Ether Ketone). Macromolecules 1989, 22, 1756−1760. (52) Marand, H.; Alizadeh, A.; Farmer, R.; Desai, R.; Velikov, V. Influence of structural and topological constraints on the crystallization and melting behavior of polymers. 2. Poly(arylene ether ether ketone). Macromolecules 2000, 33, 3392−3403. (53) Zhang, J. M.; Duan, Y. X.; Sato, H.; Tsuji, H.; Noda, I.; Yan, S.; Ozaki, Y. Crystal modifications and thermal behavior of poly(L-lactic acid) revealed by infrared spectroscopy. Macromolecules 2005, 38, 8012−8021. (54) Pan, P.; Kai, W.; Zhu, B.; Dong, T.; Inoue, Y. Polymorphous crystallization and multiple melting behavior of poly(L-lactide): Molecular weight dependence. Macromolecules 2007, 40, 6898−6905. (55) Zhang, J.; Tashiro, K.; Tsuji, H.; Domb, A. J. Disorder-to-order phase transition and multiple melting behavior of poly(L-lactide) investigated by simultaneous measurements of WAXD and DSC. Macromolecules 2008, 41, 1352−1357. (56) Zhou, S.-Y.; Huang, H.-D.; Ji, X.; Yan, D.-X.; Zhong, G.-J.; Hsiao, B. S.; Li, Z.-M. Super-robust polylactide barrier films by building densely oriented lamellae incorporated with ductile in situ nanofibrils of poly(butylene adipate-co-terephthalate). ACS Appl. Mater. Interfaces 2016, 8, 8096−8109.

I

DOI: 10.1021/acssuschemeng.6b00784 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX